WO 2005/004955 disclosed a pen-like injection device for administering a fluid product. The device comprises an apparatus for contactless measurement of a position between at least two elements that are movable relative to each other. The measurement apparatus encompasses at least two optical sensors that are placed across from each other in a stationary manner on at least one first element and a second element which is movable relative to said at least one first element. A surface profile is provided on the second element, said surface profile supplying a different predetermined profile pattern for each of the optical sensors when the first element and the second element are moved relative to each other.
Thus, according to the method suggested in WO 2005/004955 the optical sensors record different predetermined profile patterns along the surface profile when the first element is moved relative to the second element. The recorded data from the optical sensors are jointly processed in order to determine the relative position between the elements.
Thus, the apparatus for contactless measurements suggested in WO 2005/004955 applies the recoding of different predetermined profile patterns in order to determine the relative position of the two elements. The use of profiles makes the apparatus and method suggested in WO 2005/004955 rather complicated in that such profiles increase the dimensions, especially the diameter, of the pen-like device.
It is an objection of the present invention to provide an optically-based read-out mechanism for determining the position, relative or absolute position, of a dose setting member in a pen-like injection device.